A modified infant mouse assay for bacterial enterotoxins

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Abstract
The reproducibility of the infant mouse assay for Escherichia coli ST and heat-labile enterotoxins of Aeromonas spp. is improved if both intestinal weight to remaining body weight ratio (IW/RBW) and the amount of diarrhoea produced are considered as criteria for classifying enterotoxigenic strains. Animals with profuse diarrhoea may have IW/RBW ratios below the widely accepted critical value for a positive test. Using pools of supernatants from broth cultures of three different strains of E. coli, 15% of ST producers would have been regarded as negative using IW/RBW ratio as the only criterion of a positive test. In testing single supernatants, 25% of ST producing E. coli would not have been correctly classified using IW/RBW alone. A scoring system which incorporates IW/RBW ratios and the amount of diarrhoea produced improves the usefulness of the test by allowing clear separation of positive and negative strains. The scoring system is also applicable to older mice for assay of E. coli ST so that a wider age range of mice can be used allowing increased use of animal facilities.

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